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SubjectRe: [RFC PATCH 1/1] epoll: use rwlock in order to reduce ep_poll_callback() contention
On 2018-12-05 17:38, Jason Baron wrote:
>
> I think it might be interesting for, at least testing, to see if not
> grabbing
> wq.lock improves your benchmarks any further? fwiw, epoll only recently
> started
> grabbing wq.lock bc lockdep required it.

That's easy! I've just tested with the following hunk applied to my
patch on top:

+++ b/fs/eventpoll.c
@@ -1228,7 +1228,7 @@ static int ep_poll_callback(wait_queue_entry_t
*wait, unsigned mode, int sync, v
break;
}
}
- wake_up(&ep->wq);
+ wake_up_locked(&ep->wq);
}

Run time:

threads w/ wq.lock w/o wq.lock
------- ---------- -----------
8 8581ms 8602ms
16 13800ms 13715ms
32 24167ms 23817ms
No big difference. According to perf the contention is on read lock and
on try_to_wake_up(), the p->pi_lock, which serializes access exactly
like
vanished wq.lock.


- 24.41% 5.39% a.out [kernel.kallsyms] [k] ep_poll_callback
- 19.02% ep_poll_callback
+ 11.88% _raw_read_lock_irqsave
+ 5.74% _raw_read_unlock_irqrestore
- 1.39% __wake_up_common
- 1.22% try_to_wake_up
+ 0.98% _raw_spin_lock_irqsave


--
Roman

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